The invention concerns a process for the ink control or regulation of a printing machine having a colorimetric measuring system, whereby measuring fields on sheets printed by the printing machine are optically evaluated, in order to determine the color location of a measuring field in a color coordinate system and to produce a regulating value for the adjustment of the color control elements of the printing machine by coordinate comparison from the color deviation of the measuring field evaluated from a given desired color location, so that undesirable color deviations will become minimal on the sheet subsequently printed with the new ink control setting.
A process of the aforementioned type is already known from EP-A No. 228 347 (corresponding to U.S. application Ser. No. 939,966 filed Dec. 10, 1986 and Ser. No. 213,000, filed June 29, 1988), in which for the optimum matching of the color effect a plurality of reference fields are evaluated, in order to compare the color location of the reference field scanned with a color location predetermined for said reference field and to determine a layer thickness variation control vector from the color deviation between the actual color location and the desired color location, whereby the ink control elements of the printing machine are adjusted so that the smallest possible color deviation is achieved. However, occasionally it is not possible in view of certain predetermined boundary conditions, in particular given minimum and/or maximum layer thicknesses of the printing inks, to shift the actual color location to the predetermined desired color location. In such cases a color deviation error, which can be perceived to a greater or lesser extent, remains since the predetermined desired color location is outside the correction color range, the dimensions of which are determined by the permissible variation of the layer thicknesses or full tone thicknesses of the printing inks involved.
An apparatus and a process for the determination of the necessary screen surface coverage of color extracts for providing reproduction of a given master pattern to be printed with the highest possible accuracy, is described in EP-A- No. 124,908. The known apparatus comprises a measuring head, which for example contains filters for the colors red, green and blue. The apparatus makes it possible to measure color information with the use of said filters, in particular color densities of the masters scanned. The measuring head is connected with a data processing apparatus equipped with a keyboard used in the scanning of the given reference patterns for the entering of screened surface coverage values in percentages. The data processing apparatus is further provided with a display device to display the screened surface coverage values calculated on the basis of the scanning of a matter pattern.
Before it is possible to use the known apparatus for the determination of screened surface coverages of a set of color extracts, it is necessary to prepare a conversion table for the conversion of color information into screened surface coverage values, which are then stored in a memory of the data processing apparatus. For this purpose, a color sample card is initially printed. The colors cyan, magenta, yellow and black are used in the printing of the color sample card, wherein the screened surface coverages are applied between 0% and 100% in steps of 10% each, for all of the colors. This yields 14,641 combinations for the screened surface coverages and the corresponding color information, expressed for example as ink densities.
During the scanning of each pattern of the color sample card, the combination of the screened surface coverages used is entered by means of the keyboard and correlated with the color densities determined by the measuring head.
If the conversion table is applied, the apparatus is able to scan a sample pattern to be printed by means of the measuring head and to determine, by comparing the color densities measured using the assistance of the different filters with the color densities stored in the conversion table, the particular line in the conversion table having color density values which coincide with the measured color densities of the master pattern or provide the best agreement. When this line has been found in the conversion table, the correlated degrees of screened surface coverage, for example, three or four color extracts, are displayed on a display device or passed to an external device.
Because the screened color coverages for the printing were changed in steps of 10% in the preparation of the color sample card, the conversion table is relatively coarse and inaccurate. For this reason, according to an improved process, additional intermediate values are determined for color information and the correlated screened surface coverages by the interpolation of values of the conversion table. The interpolation may be carried out in a manner such that grid steps of 1% are provided, which results in a more accurate reproduction of the master pattern to be printed.
In the data processing apparatus, the color deviations between the color information of the master pattern and the color information contained in the conversion table are determined by computation to determine the screened surface coverages. The known process may also be effected so that prior to the output of the values for the degrees of screened surface coverage, a query is carried out relative to whether values of 0% or 100% are present. By extrapolating the degrees of screened surface coverage and the color densities, an extended color range for screened surface coverages between -10% and 110% is determined on the basis of the color density variations in a zone of 0 to 10% and 90 to 100%. The known process makes it possible in this manner to produce a statement concerning the lack of reproducibility of a master pattern.